The present invention relates to a process for the catalytic hydrogenation of aromatic nitro compounds and the working up by distillation of the aqueous amine solutions obtained thereby. In this process, it is possible to substantially free the amine of water in an energy-efficient manner and also to obtain the water free of amine and low boilers and to obtain the low boilers.
It is known, for example from EP-A-0 223 035, that aromatic diamines, such as, for example, toluene diamine (TDA, diaminotoluene), can be prepared by catalytic hydrogenation of the corresponding aromatic dinitro compounds. The hydrogenation can be carried out with the concomitant use of solvents, e.g., low-boiling alcohols such as methanol, ethanol or isopropanol. The hydrogenation is carried out with the aid of catalysts dispersed in the reaction mixture. The dispersed catalysts are subsequently separated off by filtration or sedimentation and are optionally fed back into the process. The hydrogenation reaction is highly exothermic. A constant problem in the hydrogenation of, for example, dinitrotoluene (DNT) to toluenediamine (TDA) is, therefore, not to dissipate this heat as waste heat but to use it expediently. Accordingly, WO-A-96/11052 describes a reaction apparatus for carrying out sludge phase hydrogenations using the heat of reaction to produce usable steam. However, a sufficiently high temperature is essential for the usability of the steam, which in turn requires a correspondingly high reaction temperature.
In EP-A-0223035, for example, steam is produced in a range from 5 to 30 bar. Taking into account the conventional temperature gradients in heat exchangers, a reaction temperature of 160° C. or higher is required to produce such steam. In EP-A-0223035, temperatures in the range from 170 to 250° C. are mentioned.
Working up of the reaction mixture obtained in the hydrogenation has been carried out by freeing a mixture of aromatic diamines and water of reaction that is obtained after removal of the solvent optionally used of water continuously in a distillation column under normal or excess pressure, and then, in further process steps, optionally, freeing the diamine obtained as the distillation residue of adherent water and of any organic compounds that may still be present. Mixtures of water with organic secondary products that are volatile in steam, as are obtained in the hydrogenation of aromatic dinitro compounds, are always obtained as distillates in this procedure. Such secondary products are, for example, aromatic or cycloaliphatic alcohols diaminotoluene, toluidines, methylcyclohexylamines and methylcyclohexyldiamines and/or methylcyclohexanol are being prepared.
The effect of these secondary products that are volatile in steam is that the water distilled via the head is greatly loaded with these compounds. A process for the working up of such aqueous amine solutions by distillation is described in EP-A-0236839. In this disclosed process, the waste water obtained is far less loaded with organic contaminants. To this end, the mixture is separated in a distillation column with sidestream withdrawal. The exhaust vapors of the distillation column are condensed, and the liquid phase obtained thereby is passed over a phase-separation apparatus in which organic secondary products that are volatile in steam are removed from the exhaust vapor condensate as the organic phase. The aqueous phase is fed back at the head of the distillation column. The water, largely freed of organic contaminants that are volatile in steam, is withdrawn via a sidestream. The diamines freed of water and contaminants that are volatile in steam are obtained as the bottom product.
A common feature of all the above-mentioned processes is that the separation by distillation of the water from the TDA- and water-containing reaction mixture obtained in the hydrogenation is carried out at normal or excess pressure, so that the heat obtained in the hydrogenation cannot be used to a substantial degree for the distillation of the TDA- and water-containing reaction mixture due to the temperature level of the steam that is produced. The above-mentioned processes therefore have a high energy consumption in the form of heating steam. From 1.2 to 2 kg of heating steam must be used per kg of water that is to be separated off.
A process that manages with 30 to 50% less heating steam than the other processes of the prior art is described in EP-A-1602640. In this process, a two- or multi-stage column arrangement is used. The pressures and temperatures of the columns are so chosen that the evaporator of the preceding or subsequent column can be operated with the heat released in the condensation of the exhaust vapor stream of a column. For example, an arrangement of two columns is shown, in which the first column is operated with an absolute head pressure of 0.6 bar and the second with an absolute head pressure of 3 bar. The evaporator of the first column is heated with the exhaust vapors of the second column. Major disadvantages of this process, however, are the increased investment needed for an additional column and the pumps, pipelines, instruments, etc. required for operation, as a result of which the economy of the process is reduced. The consumption of heating steam per kg of water that is to be separated off is accordingly about 0.7 to 1.4 kg of heating steam.